This relates to ambient sensing devices such as ion sensitive and chemical sensitive devices and to methods of manufacturing such devices that are especially suited to the manufacture of multi-element devices having sensitivity to different ions or chemicals.
It frequently is necessary to monitor the composition of a chemical environment, for example, to regulate chemical or biochemical processes, to determine air or water quality, or to measure parameters of interest in biomedical, agricultural or animal husbandry disciplines. One means of the detection, measurement and monitoring of the chemical properties of a substance involves the measurement of potential difference between two electrodes with the potential difference being dependent upon the chemical activity being measured. Because of the nature of the chemical environment, it is desirable that any measurement apparatus have at least some of the properties of: low cost, simple fabrication methodology, digital operation, some degree of signal preconditioning or intelligence, small size, high chemical sensitivity with selectivity, multiple species information with specificity, choice of reversible or integrating response to chemical species, temperature insensitivity or compensation and low power operation. In addition the measurement apparatus should have good long term electrochemical stability, good physical resiliency and strength and good resistance to corrosion and chemical attack. In the case of electrical measurement devices, the devices should also have low electrical impedance to provide good signal to noise ratios and preferably a Nernstian response to the chemical phenomena being measured.
Bergveld has proposed that hydrogen and sodium ion activities in an aqueous solution be measured by a metal oxide semiconductor field-effect transistor (MOSFET) modified by removal of the gate metal. P. Bergveld, "Development, Operation, and Application of the Ion-Sensitive Field-Effect Transistor as a Tool for Electrophysiology" IEEE Transactions of Biomedical Engineering, Vol. BME-19, pages 342-351 (September, 1972). In particular, if a MOSFET with no gate metal were placed in an aqueous solution, Bergveld suggested that the silicon dioxide insulation layer would become hydrated and then, because of impurities in the hydrated layer, ion selective. After hydration of the insulation layer of the MOSFET, Bergveld believed the device could be used for ion activity measurement by immersing the device in the solution in question and then recording conductivity changes of the device. Thus, the Bergveld device is commonly referred to as an ion-sensitive field effect transistor (ISFET).
Bergveld's work led to other developments in the field of ion sensitive electrodes such as the chemical sensitive field effect transistor (CHEMFET) device described in U.S. Pat. No. 4,020,830 which is incorporated herein by reference. As described in the '830 patent, the CHEMFET is a MOSFET in which the gate metal has been replaced by a chemically selective system that is adapted to interact with certain substances to which the system is exposed. Thus as shown in FIGS. 1 and 2 of the '830 patent, the CHEMFET is identical in structure to a MOSFET except for a membrane 38 that is deposited in place of a metal gate layer on the oxide insulator above the channel region of the transistor and, optionally, an impervious layer 44 that covers all other parts of the CHEMFET that might be exposed to the solution. Numerous variations on CHEMFET structures are disclosed, for example, in U.S. Pat. Nos. 4,180,771, 4,218,298, 4,232,326, 4,238,757, 4,305,802, 4,332,658, 4,354,308, 4,485,274 and 4,397,714. Further improvements in these structures are disclosed in application Ser. No. 441,902.
One continuing problem has been the need to provide multi-element probes. Study of chemical phenomena invariably requires more than one electrode. Even if only one parameter is being monitored by a single electrode, a reference electrode is also needed. However, because ionic concentrations are a function of pH, pH monitoring is also needed; and it is generally necessary to monitor for the presence of other ionic concentrations that might interfere with the measurement of primary interest. Because concentrations vary with space and time, it is also desirable to perform all measurements at the same time and as close together as possible. Despite the obvious need for measurements of multiple species and speculation about building such devices on monolithic structures as in S. Pace, "Surface Modification and Commercial Applications," Sensors and Actuators, Vol. 1, pp. 475 (1982), the technological difficulties associated with the marriage of different ion selective membrane materials and methods have to date thwarted any development of a significant multi-element technology. For example, no solution has been offered in the prior art for patterning a multiplicity of chemically sensitive materials such as plastics, gels and ceramics on one substrate without cross-contamination of the materials in contact. Although a two-specie probe has been described by M. Esaski et al., "Integrated Micro Multi Ion Sensor Using Field Effect of Semiconductor," IEEE Trans. Biomed. Eng., Vol. BME-25, No. 2, pp. 184-192 (March 1978), this device uses the same technology as a single specie probe with one chemically sensitive layer being formed by standard photo-lithography and the other by dip coating from solution.